1. Field of the Invention
The present invention relates to a control of an AC(Altemating Current) motor, and in particular to a control device for an AC motor which can prevent an overvoltage from being applied to the motor by minimizing power consumption of a reactor during a starting or accelerating operation of the motor, and by restricting a sharp increase of a motor line voltage resulting from a high-speed switching of an inverter during a high-speed operation of the motor, and a voltage reflection phenomenon of input terminals of the motor generated due to non-matching of a characteristic impedance between a power line and the motor, by inserting the reactor having a bar shaped core between the inverter and the motor.
2. Description of the Background Art
A PWM (Pulse Width Modulation) control method has been generally used for controlling an AC motor. The PWM control method has many advantages in controlling a switching of a power switching device used for an inverter. However, a voltage applied to the motor is sharply increased due to a voltage variation rate dV/dt resulting from a high-speed switching. Thus as a disadvantageous overvoltage is applied to the motor, so that insulating, over heating and EMI (Electromagnetic Interference) may be generated in the motor.
The causes of insulating and heating in the PWM control method can be organized into two categories. One is insulating and heating generated by a sharp increase of a line voltage due to a high-speed switching of the power switching device, and a voltage reflection phenomenon of input terminals of the motor resulting from non-matching of a characteristic impedance between a power line and the motor. The other is insulating and heating caused by a voltage (hereinafter, referred to as `common mode voltage`) generated between a motor frame and a coil due to an instant voltage unbalance in the PWM control operation.
Here, the motor line voltage is considerably influenced by the insulating and heating resulting from the voltage reflection phenomenon of the line voltage. In order to overcome this disadvantage, there has been used a method of restricting a rapid increase of an output voltage of the inverter by providing a closed type line reactor and an LRC (Coil Resistor Capacitor) filter between an output terminal of the PWM inverter and the motor.
FIG. 1 is a block diagram illustrating a conventional control device for a three phase AC motor. As shown therein, an alternating current three phase power source 1 is rectified by a converter 2 via booster reactors/inductors L.sub.R, L.sub.S, L.sub.T for each phase, and smoothed by smoothing condensers/capacitors C1, C2. Accordingly, a converted direct current voltage is applied to an inverter 3. The inverter 3 converts the direct current voltage into a three phase alternating current voltage of a variable frequency, namely an U, V and W phase alternating current voltage, and outputs it under the control of a pulse width modulation signal generator 5. The three phase U, V, W alternating current voltage is supplied to a motor 7 via the closed type line reactor 6 and a power line PL. The inverter 3 includes a plurality of power transistors (not shown) for high-speed switching, for example, IGBT (Insulated Gate Bipolar Transistor). The pulse width modulation signal generator 5 outputs a pulse width modulation signal to the power transistors of the inverter 3, and the controller 4 controls the pulse width modulation signal generator 5.
FIGS. 2A to 2D illustrate structures of the conventional closed type line reactor 6. FIG. 2A shows a single phase closed type line reactor. As shown therein, a narrow clearance or gap is formed at a core of the closed type line reactor. When a current or voltage is transmitted from the inverter 3 through the coil, a magnetic flux is formed in a clockwise or counterclockwise direction through the core. A pattern of the magnetic flux is varied according to whether a frequency of the voltage and current outputted from the inverter is high or low. In general, the low frequency is below 100 kHz, and the high frequency is over 100 kHz. FIG. 2B shows a pattern of the magnetic flux in the case that the current supplied to the single phase closed type reactor is a low frequency. As shown therein, the magnetic flux generated by the low frequency current flows through the core and the clearance. That is, a closed circuit of the magnetic flux includes the clearance. To the contrary, FIG. 2C shows a pattern of the magnetic flux when the current supplied to the single phase closed type line reactor is a high frequency. As shown therein, the magnetic flux generated by the high frequency current is mostly leaked from the portion wound with the coil, and thus does not pass through the clearance. That is, the magnetic flux passing perpendicularly to a direction of an electric current is mostly a leaked magnetic flux. On the other hand, FIG. 2D shows a three phase dosed type line reactor.
FIG. 3 illustrates an equivalent circuit to FIG. 1. As depicted therein, the dosed type line reactor 6 is represented by a serial connection of an inductance L.sub.L, and a resistance R.sub.L. The power line P.sub.L is represented by a serial connection of an inductance L.sub.P and a capacitance C.sub.P, and the motor 7 is represented by a serial connection of a capacitance C.sub.M and a resistance R.sub.M .
As shown in the equivalent circuit diagram of FIG. 3, the closed type line reactor 6 having the inductance L.sub.L and the resistance R.sub.L is provided to the power line PL having the inductance L.sub.P and the capacitance C.sub.p, and thus the closed type line reactor 6 and the power line PL serve as an LRC series circuit. Here, the inductance L.sub.L of the closed type line reactor 6 and the capacitance C.sub.M of the motor 7 become parameters for deciding a rising time of the line voltage, and the resistance R.sub.L of the closed type line reactor 6 becomes a parameter for deciding a maximal value of the line voltage.
As described above, the inductance L.sub.L element of the closed type line reactor 6 is added to the power line PL, thereby reducing the voltage reflection phenomenon resulting from the increase of the rising time of the voltage, namely controlling a rising of the output voltage from the inverter 3. In addition, the resistance R.sub.L element of the closed type line reactor 6 is added to the power line PL, and thus performs a damping operation. Accordingly, a maximum value of the line voltage may be decreased. As a result, when the closed type line reactor is designed by properly selecting the inductance L.sub.L and the resistance R.sub.L, the rising of the output voltage from the inverter may be restricted, and the maximum value of the line voltage can be decreased.
However, the closed type line reactor 6 has the following disadvantages.
Firstly, in general, the impedance of the closed type line reactor is preferably approximately 3 to 5% of the impedance of the motor. The greater the impedance value of the closed type line reactor is, the more the restriction effect of the output voltage is increased. However, there is a limit to increase the impedance value. As the amount of current is increased, a cross sectional area of the closed type line reactor, namely a cross sectional area of the core must be increased, and thus a size and a weight thereof are also increased, which results in undesirable large size and high cost. Especially, when the motor is driven at a high frequency, the magnetic flux passes perpendicularly to the direction of the electric current merely at the portion wound with the coil in the closed type line reactor. Therefore, the other portions where the magnetic flux does not pass perpendicularly to the direction of the electric current are unnecessary.
Secondly, since the closed type line reactor must be connected to the motor in series, the output voltage of the inverter is decreased by the closed type line reactor. Accordingly, the inverter must supply a voltage consumed in the closed type line reactor, in addition to the voltage for driving the motor. Especially, the motor is operated by a high frequency alternating current when reaching to a rated speed. When the motor is operated by the high frequency alternating current, the voltage generated by the portions of the whole core where the coil is not wound is lost.
FIG. 4 depicts voltage and current characteristics of each unit when the motor is driven. As shown therein, a large amount of current I.sub.M flows in the motor during a starting and accelerating period T. The inverter must supply a voltage V.sub.M for driving the motor and a voltage V.sub.L consumed in the closed type line reactor, and thus an output voltage V.sub.l from the inverter must be increased. However, a maximum value of the inverter output voltage V.sub.l is determined by a DC link voltage. Accordingly, when driven by the rated voltage, if the motor is operated at a high speed or torque, a large amount of current flows. As a result, a voltage drop of the closed type line reactor is increased, and thus a voltage necessary to drive the whole system may be greater than the inverter output voltage. In this case, the motor may not be able to output a wanted output.